The present invention relates to a transfer device for use in a retention type electrophotographic copying machine in which an electrostatic charge image once formed on a charge retentive member is repeatedly subjected to developing and transferring steps in succession to form a plurality of copies.
The applicant has developed such a retention type copying machine. FIG. 1 is a schematic diagram showing an embodiment of the copying machine of retention type developed by the Applicant and described in, for example, U.S. Pat. No. 4,215,931 issued on Aug. 5, 1980.
In FIG. 1 a sheet-like original 1 to be copied is placed on an inclined original table 2 and moved into an original feeder 3 in a direction shown by an arrow a, and feeding rollers 4 and 5 feed the original toward an original tray 6 while causing the original to pass through a transparent glass plate 7A above an illuminating lamp 7B, a reflection mirror 7C and an optical system 8. The optical system projects an image of the illuminated original 1 onto a rotary photosensitive drum 9. The photosensitive drum has a photoconductive layer 9A which, for instance, consists of selenium (Se) applied on a conductive substrate 9B. The photosensitive drum rotates in a direction shown by an arrow b. After removing charge by a charge removing lamp 10, the photoconductive layer is uniformly charged by a corona discharge device 11, so that upon projection of the aforesaid image of the original, an electrostatic latent image is formed on the photoconductive layer. This latent image is developed by a development device 12 which makes use of dry two-component developing agent, and then the developed image is forwarded to a toner-transfer station 13 as the photosensitive drum rotates.
On the other hand, a recording paper cassette 14 carries plural sheets of recording paper 15, and a rotary pickup roller 16 journaled to a swingable arm picks up the recording paper one sheet at a time, so as to feed the record paper to the toner-transfer station 13 at predetermined timing under the control of register rollers 17A and guide members 17B constituting a paper feed path 17. The toner-transfer station has a biased transfer roller 19. The recording paper 15 passes between the photosensitive drum 9 and the transfer roller 19 so as to overlay the toner image onto the recording paper to transfer the toner image thereon. During this process, the recording paper moves together with the toner image; i.e., in tight contact with the surface of the photosensitive drum, until separating pawls 21 separate the recording paper from the drum in cooperation with an air flow to be described hereinafter. A guide 17C directs the recording paper to feeding rollers 23 which feed the recording paper to a thermal fixing device 24 having a heater for fixing the toner image. Discharge rollers 25 discharge the recording paper with the fixed toner image out of the copying machine onto a copy tray 26. Since the toner image on the photosensitive drum is not completely transferred to the recording paper and partially remains on the drum, a rotary cleaner brush 27 brushes off residual toner from the photosensitive drum after the toner image passes through the toner-transfer station. A fan 28 generates an air flow to suck the brushed-off toner, and a filter 29 collects the toner particles from the air flow. A housing 30 encloses the cleaner brush and fan to produce an effective suction for sucking the toner and to prevent the toner particles from being scattered in the apparatus. The exhaust from the fan is guided by a duct 31 having an outlet facing the toner-transfer station 13, so that the exhaust air flow from the outlet of the duct coacts with the separating pawls 21 in separating the recording paper 15 from the photosensitive drum 9 in a reliable fashion.
A support pin 32 swingably carries one end of an arm 33, and the opposite end of the arm rotatably holds the rotary cleaner brush 27. The cleaner brush is kept away from the photosensitive drum when the electrostatic latent image once formed on the photosensitive drum is repeatedly subjected to development and transferring for forming a plurality of copies of one original in the retentive manner. A trimming lamp 34 is provided to face the photosensitive drum in the proximity thereof at a position between the image projecting optical system 8 and the developing device 12, so as to remove the electric charge from blind areas or those areas of the photosensitive drum which do not intend to transfer any image to the recording paper. Switches 35 and 36 detect the positions of each original in the electrophotographic apparatus, so as to sequentially control the aforementioned constitutional parts of the apparatus. Further, at the copy discharging outlet of the copying machine is arranged a copy detection switch. When copying a thick manuscript such as a book, a cover 37 of the document feed device 3 is turned about a shaft 38 in a direction shown by an arrow c so as to form a planar document feed path for a transparent book carrier on which the book to be duplicated is placed.
The transfer roller 19 comprises a conductive shaft 19A and a semiconductive rubber roller 19B arranged around the shaft and the roller is connected to a transferring bias voltage source 20.
The transfer device 18 comprising the above mentioned biased transfer roller 19 is generally used in an ordinary copying machine of single copy type in which a single copy is formed from a single electrostatic charge image. In such a single copy type copying machine, the bias voltage higher than 1000 volts, preferably 2000 volts is applied to the transfer roller, because the higher the bias voltage is, the higher the transfer efficiency becomes. However, in the copying machine of retention type, when such a high bias voltage is applied to the transfer roller, the following drawbacks might occur.
When the high transfer bias is used and a greater amount of charge is applied on a rear surface of an image receiving paper, a discharge phenomenon might be produced when the paper is peeled off the photosensitive drum. When the discharging phenomenon occurs, the toner image transferred onto the record paper is damaged and the image quality of the duplicated copy deteriorates. This drawback equally occurs in the copying machine of the retention type as well as in the copying machine of the single copy type.
In the copying machine of the single copy type, the above undesired phenomenon can be reduced by decreasing the transfer bias voltage to some extent. However, in the retention type copying machine, even if the bias voltage is made lower, there is still produced a small amount of discharge between the drum and the paper upon peeling of the paper off the drum. These small discharging phenomena generate noise charge in the latent image. The noise charge is developed during a next developing step and the developed noise images are transferred onto a next record paper. In this manner, the copies other than the first one might include undesired dots in their backgrounds.
Secondly, when the record paper is damped under a high humidity condition and becomes less resistive, electrostatic charge injected from the transfer roller 19B travels easily in a direction of thickness of the paper and deposited on the photosensitive drum. It is apparent that the deposited charge forms undesired dots in the backgrounds of all copies after the first.
In order to avoid the above drawbacks, in the retention type copying machine, the transfer bias voltage has to be substantially lowered as compared with the single copy type machine. However, even if the bias potential is limited to a low value, when the record paper is dampened under a higher humidity condition, the efficiency of transferring the toner images onto the record papers might be severely decreased and good quality copies cannot be obtained. Causes of such an undesired phenomenon may be considered as follows. When the record paper is dampened, its resistance is decreased to a great extent and thus, a lot of charge is easily transferred from the transfer roller to the toners on the photosensitive drum through the paper, so that a sufficiently high transferring electric field could not be generated. Further, the charge might be moved in the record paper along its surface and might be conducted away into ground by means of one or more electrically conductive members which are arranged along a paper feed path and made contact with the paper, so that a sufficiently high bias voltage could no longer be applied between the drum surface and the rear surface of the paper.
In order to alleviate such a drawback, it has been known from U.S. Pat. No. 3,877,416 to connect a resistor 41 of high resistance value in series with the transfer bias voltage source 20 as illustrated in FIG. 2 and a higher transfer bias voltage is applied to the transfer roller 19 by means of the resistor 41. By means of such a measure, when the resistance of the dampened paper 15 is lowered, a voltage drop is generated across the resistor 41 due to a current flowing through the roller 19, and this voltage drop can prevent electrostatic charge of opposite polarity to that of the toners from being transferred to the toners on the charge image through the record paper 15. This solution is quite effective for the single copy tupe machine, but could hardly be applied to the retention type machine, because in the latter, the transfer bias voltage must be decreased to substantial extent as explained above and thus, if the resistor of extremely high value is connected, a bias voltage might become too small to effect the transferring operation normally occurring under conditions of high humidity.
In Japanese Patent Application Laid-open Publication No. 39,053/76 there has been proposed another solution, in which a paper feed member such as feed rollers 42 is arranged in the vicinity of the transfer roller 19 as illustrated in FIG. 3. One of the feed rollers 42 is connected to a voltage source 43 and an auxiliary bias voltage having a polarity and a magnitude the same as that of the transferring bias voltage applied from the voltage source 20 is applied to the feed roller 42.
By means of such measure, a potential on a point of the paper 15 at the feed rollers 42 is kept the same as that on a point of the paper at the transfer roller 19 and thus, when the resistance of the dampened paper is decreased, charge does not flow along the record paper althrough conductive members 44 arranged along the paper feed path are connected to ground. Therefore, the transferring bias voltage is kept high even if the dampened paper exhibits extremely low resistance. This solution is effective only for the single copy type machine, cannot be used in the retention type copying machine, because in the latter machine, a great amount of charge might be injected into the photosensitive drum 9 by means of the damped paper 15 from both of the sources 20 and 43 as shown by arrows in FIG. 3. Therefore, during the repetition of developing and transferring steps, the latent image might be damaged and undesired charge might be deposited on the background of the image and, so-called, fog might occur.
In the above mentioned known solutions, injection of charge onto the photosensitive member from the transfer roller is not taken into account, because in the single copy type machine, the deterioration of the charge image after development does not cause any trouble. On the other hand in the copying machine of the retention type, the deterioration of the latent image should be limited as far as possible and thus, the injection of charge onto the photosensitive drum can never be allowed. Therefore, the known solutions can never be applied to the retention type copying machine.